1. Field of the Invention
The present invention relates to a power supply circuit, using a series regulator, to response load current flowing to the power supply circuit and controls the fluctuation of an output voltage thereof
2. Description of the Related Art
Certain power supply circuit uses a series regulator.
FIG. 1 illustrates circuitry of a related-art power supply circuit 100 using a series regulator, as disclosed in JP-2005-196354-A.
In the related art power supply circuit 100 shown in FIG. 1, because a significant voltage difference is generated between a drain voltage of the PMOS transistor M103 and a drain voltage of a positive channel metal oxide semiconductor (PMOS) transistor M104, and an input conversion offset voltage of an error amplifier 103 increases as a result, an error in the output voltage of the power supply circuit 100 may be generated. For example, assume that an output driver transistor M105, the PMOS transistors M103, M104, M106 and M107 are the same conductive type and the same size, and are driven by the same constant current. At this time, when a gate-source voltage of the PMOS transistor M104 is set as Vgs104, a drain voltage Vd104 of the PMOS transistor M104 is calculated by the following Formula a.Vd104=Vdd+Vgs104  (a)
On the other hand, when the gate-source voltage of the output driver transistor M105 and the PMOS transistor M106 are represented respectively as Vgs105 and Vgs106, a drain voltage Vd103 of the PMOS transistor M103 is calculated by the following Formula b.Vd103=Vdd+Vgs105+Vgs106  (b)
As a result, influence of a channel-length modulation effect, which depends on drain voltage, differs between the PMOS transistors M103 and M104, which causes an offset voltage. Similarly, in negative-channel metal oxide semiconductor (NMOS) transistors M101 and M102 constituting the differential pair, the drain voltage difference therebetween is generated, causing the offset voltage.
These offset voltages change due to various factors, such as inconsistencies transistor quality occurring in the manufacturing process, fluctuations in the power supply voltage, changes in temperature, and so forth. Therefore, the power supply circuit may not supply a stable voltage.
In view of the foregoing, there is market demand for a power supply circuit that quickly responds to rapid changes in load current, has reduced power consumption, and is unaffected by inconsistencies in transistor quality occurring in the integrated circuit (IC) manufacturing process, while avoiding any substantial increase in the size of the circuit.